For the denitration of exhaust gases from stationary nitrogen oxide-producing sources such as boilers, a method for reducing nitrogen oxides selectively by using vanadium oxide as a catalyst and ammonia as a reducing agent (i.e., the SCR method) has conventionally been known and is widely employed for practical purposes ("Techniques and Regulations for the Prevention of Environmental Pollution", Volume on the Atmosphere, p. 130, Maruzen Co., Ltd.). However, in this method using the vanadium oxide catalyst, the temperature of exhaust gas needs to be raised to 300.degree. C. or above in order to achieve a practically sufficient degree of denitration. Consequently, it is necessary to install a denitrator containing a catalyst bed in the high-temperature section of the boiler (e.g., just behind the outlet of the boiler or in the heat transfer section of the boiler), or reheat cold exhaust gas and thereby raise its temperature. However, these techniques involve the following problems.
When the denitrator is installed in the high-temperature section of the boiler, various problems arise in that the overall equipment becomes complicated, the use of a heat-resisting material causes an increase in equipment cost, and workability for replacement of the catalyst bed is reduced. When cold exhaust gas is reheated, an additional heater is required, resulting in an increase in equipment cost.
Accordingly, a first object of the present invention is to provide a technique by which the denitration of exhaust gases from stationary nitrogen oxide-producing sources such as boilers can be performed at low temperatures ranging from ordinary temperature (about 5 to 20.degree. C.) to about 150.degree. C.
On the other hand, exhaust gases from road tunnels and the like are characterized in that they have a much lower NO concentration of about 10 ppm or less as compared with the concentration of nitrogen oxides in exhaust gases from boilers and the like, their temperature is in the vicinity of ordinary temperature, and they are produced in enormous volumes. Consequently, in order to remove denitrate gases from road tunnels and the like according to the conventional SCR method, the temperature of the gases must be raised to 300.degree. C. or above. This requires a huge amount of thermal energy and is unprofitable from an economical point of view.
In Japanese Patent Publication No. 41142/195, Japanese Patent Provisional Publication No. 47227/'95 and the like, there has been proposed a process in which low-concentration NO at ordinary temperature is oxidized to NO.sub.2 with ozone or the like, the resulting NO.sub.2 is adsorbed to an adsorbent, and the highly concentrated NO.sub.2 is decomposed by treatment with a reducing gas such as ammonia. However, in this process involving an adsorption step, not only the equipment is increased in size and becomes complicated, but also the use of ozone poses a new safety problem. This, it is difficult to put this process to practical use.
Accordingly, a second object of the present invention is provide a technique by which NO present in exhaust gases from road tunnels and the like and hence having a low concentration and a temperature in the vicinity of ordinary temperature can be directly reacted catalytically with ammonia and thereby decomposed to nitrogen and water.
Now, an example of exhaust gas treatment by means of a conventional exhaust gas treating system is explained with reference to FIG. 7.
In FIG. 7, reference numeral 41 designates a boiler; 42, a denitrator; 43, an air preheater; 44, a dust collector; 45, a gas-gas heater; 46, a desulfurizer; and 47, a stack.
As shown in FIG. 7, a denitrator 42 using a catalyst is installed at the outlet of a boiler 41 or the like in order to remove nitrogen oxides (NO.sub.x) present in the exhaust gas, and an air preheater 43 is installed at the outlet of denitrator 42 in order to lower the temperature of the exhaust gas to about 130.degree. C.
The exhaust gas having passed through the aforesaid air preheater 43 is dedusted in a dust collector 44, passed through a gas-gas heater 45 and then introduced into a desulfurizer 46 where sulfur oxides (SO.sub.x) are removed therefrom. Thereafter, the exhaust gas is discharged into the atmosphere through a stack 47.
As described above, in the current practical process for the removal of nitrogen oxides present in exhaust gas from boilers, there is used a denitrator 42 based on the selective catalytic reduction (SCR) method in which nitrogen oxides are decomposed to nitrogen and water vapor by using a catalyst comprising V.sub.2 O.sub.5 supported on TiO.sub.2 and a reducing agent comprising NH.sub.3. However, this process involves the following problems.
First, a reaction temperature of 300 to 400.degree. C. is required because of the performance of the catalyst. Secondly, NH.sub.3 is required for use as reducing agent. Thirdly, since the current leak level of NO.sub.x is from 5 to 40 ppm, an excess of NH.sub.3 needs to be injected for the purpose of reducing the leak level of NO.sub.x to zero.
Moreover, recent environmental standards demand that the concentration of nitrogen oxides (NO.sub.x) in exhaust gases should be reduced to a level of 1 ppm or less which is commonly known as a high-degree denitration level. In the aforesaid conventional denitration treatment based on the selective catalytic reduction (SCR) method, a marked increase in removal cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. On the other hand, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of nitrogen oxides.
Accordingly, in view of the above-described problems, a third object of the present invention is to provide a denitration system which can achieve an improvement in the efficiency of removal of nitrogen oxides present in exhaust gases as compared with the prior art.